OP02: Distribution of length of observation, in months, among first observation periods across the population

Count distribution of length of observation, in months, among first observation periods across the population. 

Input:
<None>

Sample query run:
The following is a sample run of the query.

SELECT DATEDIFF(month, observation_period_start_date, observation_period_end_date) as num_months, 
         COUNT(distinct person_id) AS num_persons 
FROM ( SELECT person_ID, observation_period_START_DATE, 
observation_period_END_DATE, rank() OVER (PARTITION BY person_ID 
        ORDER BY observation_period_START_DATE ASC) AS OP_NUMBER           
        FROM observation_period) OP1 
WHERE op_number = 1 
GROUP BY DATEDIFF(month,observation_period_START_DATE, observation_period_END_DATE) 
ORDER BY DATEDIFF(month,observation_period_START_DATE, observation_period_END_DATE) ASC
Output:
Output field list:
 Field  Description
num_month Number of month duration
num_persons Number of patients whose observation period with num_month duration

Sample output record:
 Field  Value
num_month 10
num_persons 624314
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